1. Field of the Invention
The present invention relates to initial fractional frequency offset estimation. More particularly, the present invention relates to an apparatus and method, in which a frequency offset is accurately estimated by controlling an accumulation of symbol power according to a presence or absence of DownLink (DL) symbol data, in a Broadband Wireless Access (BWA) system employing beam forming.
2. Description of the Related Art
An initial fractional frequency offset estimation process is used to increase the accuracy when estimating an initial fraction frequency offset by using an auto-correlation based on a repetition structure in which a Cyclic Prefix (CP) and a valid data sample are repeated in a DownLink (DL) data symbol.
FIG. 1 is a block diagram illustrating a conventional apparatus for estimating an initial fractional frequency offset.
In FIG. 1, one symbol is assumed to include 1152 samples among which 128 samples are Cyclic Prefixes (CP).
An Analog to Digital Converter (ADC) 101 converts an analog signal received from a Radio Frequency (RF) unit into a digital signal. A compensator 102 compensates for a Direct Current (DC) offset and an In-phase and Quadrature (I/Q) balance error which are generated in the RF unit. A Receive (RX) filter 103 performs two fold down-sampling on an input signal and then performs a low pass filtering that is required for the down-sampling. A Numerically Controlled Oscillator (NCO) 104 corrects a signal phase by receiving estimation values obtained from frequency error estimators. A Fast Fourier Transform (FFT) 105 transforms a time-domain signal into a frequency domain signal. A frame synchronizer 106 obtains initial frame timing and frequency synchronization.
A 1024-symbol buffer 110 stores 1024 input data samples included in the input signal. A 128-correlation result buffer 120 stores 128 results obtained by correlating the 1024 data samples of the 1024-symbol buffer 110 with the input data samples.
For all 1152 samples, a 1152-accumulation buffer 130 stores accumulation results by accumulating a new correlation value while not accumulating the 128 data samples of the 128-correlation result buffer 120.
When the accumulation of the 1152-accumulation buffer 130 finishes, a max power searcher 140 finds a location of a sample having the greatest value among the 1152 accumulated correlation values.
A Δf-estimator 150 finds a fractional frequency offset by using an accumulated correlation value of the sample having the maximum power.
When using the conventional technique of FIG. 1, correlation values are accumulated for each DL symbol period, and the accumulation result is used to estimate a frequency offset.
The initial fractional frequency offset estimation process is performed before synchronization is obtained. That is, the process is performed before network entry is completed through a Base Station (BS), and thus a user terminal cannot obtain information on a frame the resulted from DL-MAP data decoding. Therefore, the presence or absence of DL symbol data cannot be known by using beam forming information.
As shown in FIG. 5, data is continuously accumulated in the 1152-accumulation buffer 130. Thus, when beam forming is performed for another user terminal, and there are symbols not having data, invalid correlation values are accumulated. The accumulation of invalid correlation values results in throughput deterioration in the initial fractional frequency offset estimation.
FIG. 6 is a graph illustrating a simulation result showing throughput deterioration with respect to the number of valid symbols. The greater the number of DL symbols not having data, the greater the throughput deterioration.
Accordingly, there is a need for an apparatus and method capable of determining whether to apply beam forming and then determining valid symbols having data in order to prevent throughput deterioration of a frequency offset estimation.